1. Field of the Invention
The present invention relates to a zoom lens system suitable for an imaging apparatus using a solid-state imaging device, an imaging apparatus using the zoom lens system and a method for varying a focal length of the zoom lens system.
2. Related Background Art
Recently, an imaging device such as a digital still camera and a digital video camera using a solid-state imaging device has been increasingly getting sophisticated and compact. In such an imaging apparatus, a zoom lens system is generally used for an image-taking lens. Accordingly, with a zoom lens system, a photographer can easily take a picture with an optimum angle of view appropriate to a shooting condition.
Among these zoom lens systems, the majority of those loaded on a compact camera in particular have a zoom ratio of about three, and most of them are a negative-leading type zoom lens system in which a lens group having negative refractive power is disposed to the most object side and at least one lens group having positive refractive power is disposed to the image side thereof (for example, see Japanese Patent Application Laid-Open No 2004-037700).
The reason why a negative-leading type is popular is that despite of a simple configuration with two or three lens groups this type makes it possible to obtain a zoom ratio of about three and to permit preferable aberration correction. Moreover, this type makes it possible to move the position of the entrance pupil to the object side resulting in advantages that the diameter of the most object side lens group can be small and the position of the exit pupil can be kept sufficiently away from an imaging device.
Furthermore, since a negative-leading type zoom lens system has reversed-telephoto-type refractive power distribution on the whole, it is suitable for a wide angle of view in comparison with a positive-leading type zoom lens system. Accordingly, in order to provide a zoom lens system having an angle of view corresponding to a serious wide-angle shot and optical performance corresponding to an imaging apparatus increasingly sophisticated year after year, it is reasonable to introduce a negative-leading type zoom lens system. Then, power distribution of each lens group composing the zoom lens system, lens composition in each lens group, the position of an aspherical surface, and glass materials of each lens have to be properly selected so as to provide preferable aberration correction with respect to a wide angle of view.
However, a negative-leading type zoom lens system tends to become long in the total lens length in comparison with the focal length, so that when the zoom lens system is forcibly made compact, it becomes extremely difficult to correct aberrations. Moreover, when compactness and high optical performance of the zoom lens system are made compatible, variation in aberration upon focusing tends to become large, so that it becomes difficult to secure preferable optical performance over entire focusing range from infinity to a close-range object. Furthermore, the wider an angle of view in the wide-angle end state becomes, the larger the variation in aberration upon focusing becomes, so that it is a problem.